Materials and semi-finished products which are suitable for applications in the aerospace industry and in particular for applications in aircraft interiors need to meet a demanding property profile which is described in specifications issued by the manufacturers of aircrafts. The required property profile includes a high flame retardation and low emission of smoke and toxic gases while burning.
The materials furthermore need to exhibit good mechanical properties such as a high compressive strength.
It is another overriding requirement that materials useful for the aerospace industry need to be low-weight, i.e. the materials need to exhibit a low density.
It is also required that the curable precursors of such materials exhibit a desirable viscosity and preferably a low viscosity increase with time at ambient temperature so that they can be processed, for example, by extruders, equipment comprising pumps or other conventional application equipment.
The cured materials and the corresponding precursors which are disclosed in the prior art do not always meet this property profile to a required and/or desirable extent.
U.S. Pat. No. 5,019,605 describes, for example, a low density, fire resistant one-part epoxy composition comprising    (a) from 10 to 75 percent by weight of an organic epoxide compound having an epoxide functionality of at least 1;    (b) from 1 to 25 percent by weight of an epoxide hardener;    (c) from 20 to 50 percent by weight of a smoke suppressant;    (d) from 10 to 25 percent by weight of a filler capable of reducing the density of said composition; and    (e) from 1 to 10 percent by weight of an organic phosphorous-containing compound selected from the group consisting of alkyl phosphates, aryl phosphates, alkylphosphonates, aryl phosphonates, and mixtures thereof,wherein said composition contains from about 3 to 10 weight percent bromine.
This composition provides smoke suppression and has an excellent compressive strength but is potentially toxic while burning due to the bromine content.
EP 0,814,121 discloses processable one-part precursors of a low-density-fire-retardant epoxy-based composition which preferably also comprise a bromine source.
EP 0,693,092, EP 0,459,951 and WO 99/45,061 disclose essentially halogen-free fire-resistant epoxy compositions. EP 0,693,092 discloses the use of low-density fillers such as hollow glass microspheres in order to reduce the weight and the density of the cured epoxy compositions while maintaining a high compressive strength. A similar approach is used in EP 0,459,951 which prefers a combination of glass balloons and fumed silica as fillers. The hollow glass microspheres typically have a density of about 0.25 to 0.35 g/cm3 so that a relatively high amount of glass microspheres needs to be added to the compositions in order to provide a desirable decrease in weight and density of the compositions. This is recognized, for example, in U.S. Pat. No. 6,635,202 where it is stated that significant quantities of hollow glass microspheres need to be included into processable syntactic epoxy-based pastes in order to decrease the density of the cured paste to a value of below 0.6 g/cm3. U.S. Pat. No. '202 also discloses that a high loading with glass microspheres results in a sharp increase in viscosity so that the uncured pasty precursor requires kneading by hand or a dough mixer. It was found by the present inventors that the addition of higher amounts of glass microspheres to state-of-the-art compositions of, for example, more than 25 weight percent and, in particular, of more than 27.5 weight percent with respect to the mass of the composition tends to increase the viscosity and hence to decrease the processability of the precursor of such conventional epoxy-based fire-retardant compositions to an undesirable and often not acceptable degree.
U.S. Pat. No. 6,635,202 discloses a thermally expanding epoxy-based powder that can be used as a void-filling material for honeycomb sandwich panels which are used in the aerospace industry. The powder comprises an epoxy compound, an epoxy hardener and a filler component which comprises an unexpanded thermoplastic microsphere, optionally in combination with an inorganic filler such as fumed silica. The powdery precursor is obtained by mixing the starting compounds at temperatures below the onset temperature of the epoxy compound and below the temperature where expansion of the unexpanded thermoplastic microspheres occurs. The powders of U.S. Pat. No. '202 are filled in a honeycomb structure and heated thereby expanding, foaming and curing the powder.
The powders of U.S. Pat. No. '202 cannot be used in vertical bonding geometries which limits their applicability. Foaming adhesives can furthermore only be used in specific applications. The compressive strength of the cured powders of U.S. Pat. No. '202 does furthermore not always meet practical requirements.
It was therefore an object of the present invention to provide a low-weight fire-retardant composition which when burning has a low emission of smoke and toxic gases and which is versatile in its technical applicability. It was another object of the present invention to provide a flame-retardant epoxy composition which upon curing exhibits favourable mechanical properties such as, in particular, a high compressive strength and a low density of preferably not more than 0.6 g/cm3 while the precursor of such composition is characterized by a good or at least acceptable processability such as, in particular, a good extendibility. Other objects of the present invention can be taken from the following detailed specification.